For mobile phones, batteries with a higher capacity are desirable. Since it is difficult to increase the capacity of secondary batteries, direct methanol fuel cells using methanol fuel become of greater interest.
While polymer electrolyte fuel cells (PEFCs) functioning on hydrogen fuel generally use Nafion (trademark, DuPont) membranes, the performance of DMFC is evaluated using the Nafion membrane.
Exemplary DMFCs using Nafion membranes include the following.
(1) Li Liu et al., Electrochimica Acta 43 (24), 3657-3663, 1998,
membrane type: Nafion 117
anode catalyst type: PtRu/C, PtRu black,                PtRu loading 0.26-6 mg/cm2         
cathode catalyst type: Pt black, Pt loading 6 mg/cm2 
(2) C. Y. Chen et al., Journal of Power Sources, 141, 24-29, 2005
membrane type: Nafion 117
anode catalyst type: 60 wt % PtRu/C, PtRu black,                PtRu loading 1-8 mg/cm2         
cathode catalyst type: 60 wt % Pt/C, Pt black,                Pt loading 1-10 mg/cm2         
To incorporate DMFC in mobile phones, the DMFC is desired to have a higher methanol concentration (e.g., 10M or higher) and required to produce a higher power.
The Nafion membrane used therein has a substantial methanol crossover and allows methanol to permeate therethrough, suffering from the problems of power decline and a lower utilization efficiency of methanol fuel.
Therefore, among Nafion membranes, Nafion 117 having a substantial thickness (175 μm thick) is often utilized. As further power enhancement is required, a membrane which is reduced in methanol crossover without losing ion conductivity is desirable.
The technology pertaining to the present invention is described in the following references.                Patent Reference 1: JP-A 2001-348439        Patent Reference 2: JP-A 2002-313364        Patent Reference 3: JP-A 2003-82129        Non-patent Reference 1: Journal of Applied Polymer Science, Vol. 68, 747-763 (1998)        